Cyclospora cayetanensis n. sp. is a newly recognized parasitic protozoan infection of humans in Peru. It now appears quite evident that what has been described as "cyanobacteria-like bodies" (CLB) in association with prolonged diarrheal illness worldwide, in fact, can be attributed to this new parasite. Except for prolonged intermittent diarrhea, the clinical presentation of patients infected with this organism is very similar to those infected with Cryptosporidium parvum. Interestingly, and unlike infections with c. parvum, anecdotal chemotherapeutic approaches using trimethoprim sulfamethoxazole to treat illness caused by this parasite have proven successful. Because Cyclospora is a newly described parasite from humans and is responsible for causing disease, we plan to undertake studies which will provide important new information about the epidemiology, life cycle, molecular biology and treatment of this organism. Specific approaches to be undertaken include: 1) a cohort and nested case controlled study of infection with Cyclospora in children, 2) a diarrheal survey through visitor clinics to define infection in expatriates to endemic zones, 3) development of a serologic test to define prevalence in an endemic zone, 4) a survey of community water sources to determine if water is involved in parasite transmission, 5) a double-blinded crossover drug study using trimethoprim sulfamethoxazole, and 6) oocyst purification to provide organisms for a host of studies to include: a) development of monoclonal antibodies to derive new diagnostic tests to detect infection, b) in vitro cultivation, and c) genomic analysis.